The present invention relates to gas-assisted injection molding machines and, more particularly, to injection needles for use with gas-assisted injection molding machines.
Gas-assisted injection molding is commonly used to produce hollow articles having improved structural integrity and significant weight and material reduction. According to this technique, molten molding material, such as thermoplastic or rubber, is injected within a mold cavity in a manner conventional in the art. A gas, such as nitrogen, is then injected into the mold cavity through one or more injection needles typically positioned near the molding gate area or near a thick section of the article to be molded. As the gas is injected into the article to be molded within the mold cavity, a portion of the molten molding material within the mold cavity is forced or expelled out through a vent valve. Consequently, a wall of molding material remains around the periphery of the mold cavity, thereby forming a hollow molded article having improved structural integrity and significant weight and material reduction.
Typically, injection molding machines include a core and a cavity plate mounted on a base plate or clamping plate. The core and cavity plate are separable along a common plane. The mold cavity is formed on the common plane to facilitate the removal of the molded article when the mold is opened. To further facilitate the removal of the molded article, an ejector mechanism is commonly employed. The ejector mechanism includes pins movable normal to the common plane used to elevate the molded article relative to the mold cavity when the mold is opened.
As described above, gas is typically injected into the mold cavity by at least one injection needle. Each injection needle is typically slidably disposed in the core and is spring biased into a retracted position. Upon introduction of the gas, the injection needle is raised via gas pressure into the mold cavity against the force of the spring, thereby supplying gas to the mold cavity. A complete description of a gas injector mechanism and an ejector mechanism may be found in U.S. Pat. No. 5,511,967 entitled xe2x80x9cSelf-Contained Gas Injectorxe2x80x9d (xe2x80x9c""967 Patentxe2x80x9d), which is incorporated herein by reference.
Briefly, the ""967 Patent describes, among other things, a pair of inclined slots that direct jets of gas from a pair of flats radially into a mold cavity. When the head is in a retracted position, the head seals the pair of flats, thereby preventing the return flow of gas down the needle. A return gas flow may be preferred in situations where an increased return gas flow rate is needed or in situations where a separate gas vent valve will not or can not be used. Moreover, the pair of flats formed on the needle may not afford maximum structural integrity of the needle since significant lateral movement of the needle may occur.
Additionally, in operation, the valve member of the ""967 Patent may clog when forming articles using a highly viscous or a rubber-like molding material. As should be appreciated to those skilled in the art, injection of molding material is typically accomplished at a pressure of approximately 20,000 psi. This extreme pressure has a two fold effect on the ""967 injection needlexe2x80x94first, the blowing force of the material against the ""967 injection needle forces the stem to be pushed to the side due to the pair of flats on opposing sides of the stem, which cause increased wear in the needle and reduces its operational life. Second, the molding material may be forced into the injection needle through any gaps, such as the pair of flats and/or wear gaps formed by the stem being forced to the side. When the material is forced into the injection needle, the ""967 needle is unable to clear the material and becomes clogged.
Accordingly, there exists a need in the relevant art to provide a gas-assisted injection molding needle having vent slots to enable the return flow of gas from the mold cavity through the needle to the atmosphere or into a storage chamber, which is simultaneously capable of substantially eliminating any clogging effect. Moreover, there exists a need in the relevant art to provided a gas-assisted injection molding needle that is capable of maximizing the structural integrity of the needle and minimizing the wear thereof.
In accordance with the broad teachings of this invention, an injection needle for use in a mold cavity of a gas-assisted injection molding machine having an advantageous construction is provided.
In one aspect of the present invention, the injection needle includes a tubular member and a stem slidably disposed in the tubular member. In another aspect of the present invention, the stem is positionable in a raised position and a retracted position. A further aspect of the present invention provides an injection needle that includes a central bore extending within the stem for supplying a gas. An aperture is disposed in the stem and is in fluid communication with the central bore. The aperture is operable for supplying the gas into the mold cavity when the stem is in the raised position. Yet another aspect of the present invention employs a vent slot disposed in the stem and is in fluid communication with the central bore. The vent slot is operable for relieving gas pressure within the mold cavity when the stem is in the retracted position.
The injection needle of the present invention is advantageous over traditional constructions since the present invention enables the return flow of gas from the mold cavity through the vent slot to facilitate equalization of the gas pressure in the mold cavity relative to ambient condition. Furthermore, the vent slot of the injection needle is sized to minimize or substantially eliminate backflow of molten molding material through the injection needle to minimize or substantially eliminate any clogging effect. Still further, the outlet aperture is tapered to enable gas pressure to self clean the aperture and vent slot by forcing any clogging obstruction out of the needle. The injection needle of the present invention further maximizes the structural integrity of the injection needle to provide an improved useful life. Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.